The invention relates to a process for bonding polyester reinforcement elements to rubber.
Polyester fibers, cords and fabrics are used extensively for reinforcing rubber articles such as tires, belts, hoses and the like. But, polyester textile elements generally adhere poorly to rubber unless first treated to improve adhesion, as by coating. Because of the exceptional strength and reinforcing ability of polyester fibers there has been a continuous effort to improve the adhesion of polyesters to rubber.
One widely used process for adhering polyester to rubber involves coating polyester with a resorcinol-formaldehyde latex followed by heat treating the coated polyester which is then incorporated into rubber. But, this system can be unsatisfactory depending on factors such as the degree of polyester heat treatment; the composition of the polyester substrate; the amount of stress and heat the reinforced rubber article undergoes; and the like.
Active isocyanates, particularly the polyisocyanates, have been added to resorcinol-formaldehyde latexes to further improve polyester to rubber adhesion. But, the rapid reaction of isocyanates in water and the toxicity of isocyanate compounds are shortcomings to this system. To overcome these problems, the isocyanates have been added in "blocked" form to latexes. Thus, isocyanates are reacted with blocking agents, most notably phenol, to provide compounds which are stable at normal temperatures but which, upon heat treatment at above about 100.degree. C., disassociate to form a free isocyanate and phenol. A polyester substrate is coated with the mixture of resorcinol-formaldehyde and blocked isocyanate. Upon subsequent heat treatment of the coated polyester, the isocyanate is formed, in situ. This improves bonding between the latex and polyester.
Blocked isocynates, such as alcohol or phenol blocked isocyanates, i.e. urethanes, are however, typically water-insoluble and must be added to the latex as an aqueous emulsion or as a solids dispersion. It is difficult to achieve and maintain a uniform dispersion of the urethane throughout the resorcinol-formaldehyde latex. This can lead to non-uniform adhesion of the polyester substrate to the rubber.
Polyepoxides have also been added to the latex-urethane mixtures, see U.S. Pat. Nos. 3,234,067 to Krysiak and 3,933,677 to Auftermarsh. Since these systems involve emulsions or dispersions of the urethane, as above, uniformity of adhesion can be subject to question.
It has been suggested to precoat polyester fibers prior to latex dipping, with an aqueous emulsion containing a dispersion of blocked isocyanate solids. (TBI 29; FORTREL.RTM. TYPE 900 POLYESTER INDUSTRIAL YARN; Celanese Fibers Marketing Company; October 1977, page 9). This predip does result in increased adhesion. But as with previous systems which employ solids dispersions, coating uniformity is difficult to control, and the availability of reaction sites is lessened with solid reactants.
It has also been proposed to coat polyester fibrous substrates with the combination of a polyepoxide compound and a blocked isocyanate compound which is then heated causing unblocking of the polyisocyanate and curing of the epoxide/isocyanate coating. The coated polyester is then treated in a resorcinol-formaldehyde latex which may contain other materials, followed by a second heat treatment and bonding to rubber. Such processes are disclosed in U.S. Pat. Nos. 3,272,676 to Kigane et al; 3,307,966 to Schoaf; 3,503,845 to Hollatz et al; and Japanese Patent Publication No. 62,269/1980 to Takada. The dual heat treatments required by these processes involve increased energy usage with attendant increased cost. Additionally, unblocking the isocyanate prior to exposing the polyester to resorcinol-formaldehyde bath renders the isocyanate unavailable for reaction with the resorcinol-formaldehyde.
Numerous other treatments for adhering polyester to rubber have been proposed, but none has been completely successful in overcoming adhesion difficulties. There is thus a continuing need for improving processes for adhering polyester to rubber.